@Malcolm ************
ok, one last time it's NOT a visa on arrival ๐ There is no such thing as a 30 day visa on arrival.
You are talking about a 30 day VISA EXEMPT ENTRY.
You can't extend it for 90 days but what you're wanting to do is actually APPLY for a 90 day Non-O visa for 2000baht at your immigration office (by meeting the requirements) wait the under review period (between 14 and 20 days depending on the office you use) then go back and get a new 90 day stamp AND a Non-O visa inked in
After you run that down to 30 days or less then you'd apply for a year extension for 1900baht
The whole process can be done from a 30 day visa exempt entry stamp that you get comin' in by land (y)